Influence of the silicon content upon the acidity and catalytic properties of AFI-like catalysts Original Research Article

A series of highly crystalline silicoaluminophosphates having the AFI topology (SAPO-5) were synthesized with silicon content varying from 0 (AlPO4-5) to 0.11 molar fraction. As the silicon content increases, the total acidity per gram of solid, as determined by temperature programmed desorption of ammonia (NH3-TPD), also increases. However, the number of total acid sites per Si atom decreases by about 20% when passing from the lowest to the highest Si-containing SAPO. The previous finding is in line with the higher SM2+SM3 Si-substitution character of the latter, as determined by the 29Si MAS NMR technique. Brönsted sites retaining pyridine above 623 K (medium+strong acidity) increase progressively with the Si content; however, only the solid with the lowest Si-loading showed Lewis sites of high acid strength. The highest specific activity observed for the latter sample during both the m-xylene and the α-pinene transformation can then be explained by the occurrence of a Lewis–Brönsted synergism. The specific rate of α-pinene transformation over SAPO-5 lay between that for dealuminated mordenite and that for dealuminated Y-zeolite. Camphene and limonene were the main products of the α-pinene transformation amounting up to 80% of the α-pinene converted, with a camphene/(camphene+limonene) ratio of 0.50, at conversion levels between 48 and 55%. Undesirable heavy compounds (HRTP) were less than 1%.